The present invention relates to methods and devices for the measurement of temperatures and air and oxygen pressures with a single paint, optical fiber or other probe, and more particularly to said methods and devices using photoluminescent probes.
The measurement of oxygen pressure using photoluminescent dyes has been known for decades. The 1971 U.S. Pat. No. 3,612,866 to Stevens describes a method for determining oxygen concentrations from the quenching of the photoluminescence of the hydrocarbon pyrene embedded in oxygen-permeable plastics. Bacon and Demas used polymer-immobilized ruthenium complexes with for the same purpose [Anal. Chem. 59, 2780-85 (1987)]. U.S. Pat. No. 4,810,655 to Khalil and Gouterman provide a historical background referencing work done up to about 1986, including but not limited to the use of these and other photoluminescent materials, notably platinum porphyrins, at the tip of optical fibers for measuring oxygen pressure in blood. U.S. Pat. No. 5,965,642 to Gouterman and Carlson update that account to about 1997 and also describe the use of oxygen-sensitive photoluminescent dyes as paints used for mapping air pressure distributions on aerodynamic surfaces in wind tunnel studies. All of the above references use photoluminescent indicators so characterized that, when excited by a pulse of light of microsecond or sub-microsecond duration and wavelength or wavelengths within their lowest energy electronic absorption band, they emit a luminescence light with a decay time xcfx84ox which decreases in a known manner with increasing oxygen pressure. The decrease xcfx84ox parallels the quenching effect of the oxygen pressure. If xcfx84 is the luminescence decay time in the absence of oxygen and tox is the decay time in the presence of oxygen, then xcfx84/xcfx84ox=I0/Iox, where I0 is the luminescence intensity in the absence of oxygen and Iox is the lower luminescence intensity in the presence of oxygen.
There is a need, in a plurality of fields, to measure simultaneously or quasi-simultaneously (within one or a few seconds) both the temperature of an object or environment and a second parameter, physical or chemical. In most cases the main objective is to measure said second parameter, but its measurement is substantially affected by temperature. In clinical practice it is often necessary to measure both the oxygen pressure and the temperature of blood or a tissue with a fiber optic technique. A preferred method for measuring oxygen pressure is the use of an oxygen-sensitive photoluminescent dye. The dimensional constrains may require that the same probe be a temperature probe as well.
Demas et al. disclosed that the same ruthenium complexes used for measuring oxygen pressure can be used as temperature indicators [Proc. SPIE, 1796, 71-75 (1992)], but in order to use the complexes as temperature probes it was necessary to exclude oxygen from them. His work did not teach or anticipate a way to measure temperature while the probe luminescence was being simultaneously quenched by oxygen.
U.S. Pat. No. 6,303,386 to Klimant et al. describes a system for measuring both oxygen pressure and temperature using a probe having two sensing layers. One layer has an immobilized oxygen-sensing porphyrin. The other layer has an immobilized ruthenium complex for measuring temperature. Only the oxygen-sensing layer was permeable to oxygen. The arrangement required two light sources and associated optical filters, and two photodetectors and associated electronics.
The measurement of air pressure distributions on three dimensional aerodynamic test surfaces is one of the many applications of pressure-sensitive paints, and the preferred paint technology is still, to this date, based on the oxygen quenching of photoluminescence. One limitation of this technique is that the pressure readings are affected by temperature changes. If temperature gradients are relatively large over the test surface the prior art requires a temperature-sensitive paint in addition to the pressure-sensitive paint. In some applications where two or multiple parts of the body under study are subject to identical fields, for instance rotor blades in turbomachinery, one can separate the pressure-sensitive paint from the temperature-sensitive paint and perform independent measurements on the two paints, which requires two different sensor systems. In parts of the surface under study where both temperature and pressure readings are required on the same point, the two paints must be applied, one on top of the other. This, in addition to requiring two sensor systems, may introduce serious compatibility problems between the two paints, as one paint may interfere with the measurements performed on the other.
Prior art temperature sensing techniques for moving objects like rotor blades use a luminescent paint applied to the object and having a temperature-dependent luminescence decay time xcfx84T, which decreases in a known manner with increasing temperature as the luminescence quantum efficiency of the paint decreases. In order to get accurate data, the use of two sensing layers as in the prior art is subject to stringent requirements for the temperature sensing layer, as listed by Allison et al. xe2x80x9cA Survey of Thermally Sensitive Phosphors for Pressure Sensitive Paint Applicationsxe2x80x9d, ISA Paper 472, May 2000. They are, inter alia:
1) Very uniform coatings;
2) The luminescence decay time xcfx84T must be shorter than 10 microseconds;
3) The luminescence should be excitable with a blue emitting diode (LED);
4) The luminescence spectrum should be different from that of the pressure sensing layer;
5) The luminescence of the phosphor should not excite the pressure sensing layer to luminesce.
These requirements could be relaxed, or even eliminated, if one could measure temperature and air pressure accurately and independently of each other, but with the same indicator. That would also greatly minimize the sources of error and greatly reduce the complexity of the measuring system.
There is a need, therefore, for a simple measuring system wherein the same oxygen-sensitive photoluminescent material used as a pressure probe can be used as a temperature probe. It is also desirable that the added temperature measurement on the pressure probe do not substantially increase the complexity of the pressure measuring system or require a different dedicated temperature measurement system.
One prior art system for measuring temperature, suitable for use with fiber optic techniques and referred to herein as the Thermally Activated Direct Absorption (TADA) system, is based on the direct measurement of a temperature-dependent optical absorption, using photoluminescent probes as the absorption indicators. The system is described in U.S. Pat. No. 5,499,313 to Kleinerman, which incorporates teachings from previous patents to Kleinerman. The system is suitable for measuring temperatures at any chosen point or at a multiplicity of points along which a long optical fiber probe is deployed, but it loses accuracy in temperature ranges within which the luminescence efficiency of the probe is substantially degraded. Furthermore, nothing in that patent or any other prior art teaches how to measure temperature and another physical or chemical variable with a photoluminescent indicator which is being simultaneously affected by both variables, or how to measure surface temperature distributions with a single indicator dispersed in a non-homogeneous coating.
It is the main object of the present invention to provide simple and inexpensive optical methods and instrumentation for measuring the temperature of objects or environments in the presence of other, simultaneously acting physical or chemical variables.
It is another object of the present invention to improve the TADA system so it can be used in temperature ranges within which the luminescence efficiency of the probe is substantially degraded.
It is a specific object of the present invention to provide simple and inexpensive methods and instrumentation whereby a single probe is used to measure both oxygen pressure and temperature essentially at the same time and independently of each other.
It is another object of the invention to provide improved systems for the optical measurement of diverse physical parameters while providing temperature compensation, using a single probe.
It is another object of the invention to provide better techniques for visualizing the air pressure and temperature distributions on the surfaces of solid bodies.
Yet another object of the invention is to provide new methods and devices for obtaining accurate measurement of surface temperature distributions using an indicator dispersed in a non-homogeneous paint, even if the indicator is being simultaneously affected by another distributed physical or chemical variable.
It is a further object of the invention to provide improved methods and devices for measuring localized temperatures and surface temperature distributions on fast moving bodies.
Within the context of this application, I am using the following definitions:
Light: optical radiation, whether or not visible to the human eye.
cmxe2x88x921: energy units expressed as the inverse of the corresponding wavelength xcex when the wavelength is given in centimeters (cm).
Excitation light: illuminating light which can generate luminescence in a luminescent material.
Interrogating light: illuminating light injected into or incident on an optical probe for the physical variable.
Luminescence: Light emitted by a material upon absorption of light or other radiation of sufficient quantum energy. The term includes both fluorescence and phosphorescence.
Luminescence centers: atoms or molecules (including ions) of a photoluminescent material which absorb excitation light and emit luminescence light.
Luminescence decay time xcfx84: the time after the cessation of the excitation radiation in which the intensity of the luminescence decays from Io to Io/e, where e is equal to 2.71828 and Io is the luminescence intensity at any reference time chosen as xe2x80x9czeroxe2x80x9d time.
Luminescence quantum efficiency xcfx86 (also referred to as luminescence efficiency): the ratio of the number of luminescence photons emitted by a luminescent material to the number of photons of the excitation light absorbed by the material.
Luminescence time rate of decay: the inverse of luminescence decay time xcfx84.
Single Luminophor: a photoluminescent material, whether pure, dissolved or dispersed in a polymer matrix, a glass or a paint, having a single light-emitting species, for example a specific platinum(I) porphyrin, or a specific ruthenium(II) complex with tris(4,7-diphenyl-1,10-phenanthroline), but not a composition containing both. Other example: Nd3+ or other specific rare earth ion whether as a dopant or in a stoichiometric compound.
Occupancy number of an energy level: the fraction of the total number of molecules of a probe material occupying said energy level.
Paint: a relatively thin coating, whether or not colored, applied to an object as a sensing probe.
Photoluminescence: Luminescence generated by the absorption of light.
Physical variable: any physical (including chemical) property whose magnitude can change. Examples: pressure, temperature, flow rate, position, liquid level, oxygen and the like. (Synonims: measurand, physical parameter).
xcex1: wavelength of luminescence excitation light the optical absorption of which is not substantially affected by temperature.
xcexv: wavelength of luminescence excitation light the optical absorption of which is substantially temperature-dependent.
The present invention improves and substantially extends the scope of the temperature measurement system based on the direct measurement of a temperature-dependent optical absorption by photoluminescent probes. That system, as described in section 2.1 of U.S. Pat. No. 5,499,313, and referred to herein as the Thermally Activated Direct Absorption (TADA) system, is based on a physical property shared by virtually all liquid or solid materials having an optical electronic absorption band in the visible or near infrared region of the optical spectrum. When these materials are illuminated with light of any wavelength or wavelengths xcexv within the long wavelength tail of their lowest energy electronic absorption band, the magnitude of the fraction xcex1 of the intensity of the light which is absorbed is temperature-dependent, increasing in a known manner with increasing temperature. If these materials are photoluminescent, the luminescence intensity generated by the absorption of light of said wavelength or wavelengths xcexv is also temperature-dependent, this intensity increasing in a manner directly proportional to the magnitude of xcex1 if the luminescence quantum efficiency of the photoluminescent material is not degraded over the temperature range of operation. A measurement of a luminescence intensity directly proportional to xcex1 is a direct measurement of light absorption, in contract to light transmission measurements, where the value of xcex1 is determined indirectly as a difference between two light intensities, not measured directly. U.S. Pat. No. 5,499,313 teaches how to measure temperature at any chosen point with a discrete sensor of known composition and thickness, and how to extend its main concept to the measurement of distributed temperatures by using a suitably doped long optical fiber probe.
The instant invention improves and substantially extends the TADA system so it can be used in temperature ranges within which the luminescence efficiency of the probe is substantially degraded.
Furthermore, the instant invention teaches new techniques for further extending the reach of the TADA system to allow a single photoluminescent probe material to be used for both temperature and oxygen and air pressure measurements, essentially simultaneously and independently of each other. Although they use the same probe, the measurements of temperature and of oxygen pressure do not interfere with each other when used according to the teachings of this invention. The reason there is no interference can be understood by noting that the oxygen quenching of the photoluminescence is a processes which occurs after the absorption of the excitation light, but the physical process indicative of the probe temperature is a light absorption process which occurs prior to the photoluminescence and is not, therefore, affected by any processes which affect the photoluminescence efficiency, provided that the photoluminescence intensity is measurable to the needed extent. This is an easily met requirement given the great sensitivity of light detectors for visible and near infrared radiation.
Now, a luminescence intensity generated by a temperature-dependent absorption of light of a given wavelength does depend on oxygen pressure, as this pressure affects the luminescence quantum efficiency in a manner that can easily be pre-determined. But this invention teaches how to make the temperature reading independent of luminescence quantum efficiency by referencing the temperature-dependent luminescence intensity to a luminescence intensity generated by absorbed light of a different pre-selected wavelength.
The technology subject of this invention can also be used for measuring temperature with any probe used for sensing concurrently any other physical or chemical measurand, provided the probe uses an efficient photoluminescent indicator, whether the indicator is unchanged, generated or partially consumed in the process.
The instant invention extends the capability of these techniques still further, by allowing the measurement of air pressure and temperature distributions over the surface of a body subject to these air pressure and temperature distributions. The invention permits these measurements with reasonably high accuracy using as sensors photoluminescent coatings even when the sensing points on the coatings are of non-uniform thickness and would, under the prior art techniques, generate many erroneous readings due to their different light absorption path lengths. The invention includes features for cancelling out the effects of these different thicknesses by performing measurements in two different wavelength regions and comparing the readings obtained from these two wavelength regions.
Still further, the instant invention makes it possible to measure accurately the surface temperatures of fast moving bodies, for example rotating turbine blades. In the prior art these measurements use as probes paints applied to the surface of said bodies, the paints including a photoluminescent material having a temperature-dependent luminescence decay time xcfx84T. The measurements are carried out by exciting the luminescence of the paint with pulses of light of microsecond or sub-microsecond duration and measuring the luminescence decay time xcfx84T. A serious disadvantage of this method is that, in a fast moving body one has to measure the intensities of two short duration fractions of the time-decaying luminescence from the illuminated spot. The first fraction is measured very shortly after the extinction of the excitation pulse, before the peak luminescence intensity has decayed significantly. The second fraction is measured a short interval afterwards, as the spot has moved rapidly away from the position where the intensity of its first luminescence portion was measured. Now, except for a relatively small group of materials described in Kleinerman""s U.S. Pat. No. 5,222,810 section 2.0: Luminescent Materials Having two Emissive Levels with Temperaturexe2x80x94Dependent Relative Populations, a decrease in the luminescence decay time xcfx84 of a probe with increasing temperature parallels a decrease in its luminescence efficiency, which inevitably decreases the signal-to-noise ratio of the measurement. In the high temperature region (above 500xc2x0 C.) within which the luminescence decay time xcfx84 decreases appreciably per increasing degree the luminescence quantum efficiencies are often of the order of 10xe2x88x922 or smaller. And since only a small fraction of the emitted luminescence intensity is measured, measurement accuracy is limited. The present invention overcomes these shortcomings and permits the measurement of temperatures and surface temperature distributions with photoluminescent probes which maintain their high luminescence efficiencies over their temperature range of operation and do not require a temperature-dependent change in their luminescence spectral distribution or luminescence efficiency.